Electric field E at a point is defined as the force per unit positive test charge: E = F/q_0 (in the limit q_0 -> 0 so it doesn't disturb the source). SI unit: N/C = V/m. Dimensional formula: [M L T^(-3) A^(-1)]. For a point charge Q at distance r: E = kQ/r^2 directed radially outward for positive Q and inward for negative Q. The electric field is a vector field — it has both magnitude and direction at every point in space. It is the mediator of the electrostatic force: instead of action-at-a-distance, charge Q creates a field, and another charge q in that field experiences force F = qE. The field exists even if no test charge is placed there.